This invention relates to a process for preparing sodium percarbonate in the form of a powder with a narrow grain size range, high apparent density and resistant to abrasion, and of which the elementary particle is a regular rhombohedral crystal.
The conventional procedure for obtaining sodium percarbonate, Na.sub.2 CO.sub.3 . 1.5 H.sub.2 O.sub.2, using the wet method, consists in mixing a solution of anhydrous sodium carbonate, Na.sub.2 CO.sub.3, and a solution of hydrogen peroxide.
The percarbonate crystallises and this phenomenon is exothermic. In order to obtain the best yields with respect to the hydrogen peroxide used, it is necessary to keep the crystalliser at a temperature equal to or lower than 15.degree. C.
This elimination of the calories liberated by crystallisation is effected by heat exchange with cold brine through the metal walls of the crystalliser and/or coils and/or various cooling arrangements. The inconvenience of this system is the tendency of the solid percarbonate to adhere to the cold walls. In order to obtain a stable product having good particle size characteristics, it is necessary to introduce into the crystalliser agents which modify crystallisation and stabilisers, such as sodium silicate, which favour the formation of precipitates; the above-mentioned encrustation rapidly reduces the exchange capacities of the apparatus and, all things being equal, the temperature rises, and this increases the speed of decomposition of the active oxygen present in the medium in the solid or light phases; as this decomposition phenomenon is itself exothermic, the heating process is automatically accelerated; the resulting yield thus becomes much too low and the apparatus functions in an irregular manner.
It is possible to overcome these inconveniences either by decreasing the quantity of heat generated in the crystalliser by reducing the rates of flow, but as a consequence thereby lowering the production capacity of the apparatus; or by effecting mechanical scraping of the walls of the apparatus, but this is not always easy to carry out in practice. Another disadvantage of this process consists in the relatively low solubility of the carbonate in the water, while that of the percarbonate is fairly high; also the use of carbonate in solution leads to the crystallisation occurring in a relatively dilute medium, which reduces production capacity and yield, because the active oxygen in an alkaline medium is of particularly low stability.
Finally, a third disadvantage of the process is the requirement of a cooling source in a relatively troublesome form, since it is only possible to lower the temperature of the crystalliser to below 15.degree. C. by using brine having a temperature which is between 0.degree. and 10.degree. C.